The invention relates to a drive for safety a belt tensioner having a drive chamber.

German Offenlegungsshcrift DE 199 61 109 A discloses a belt tensioning safety belt system which is particularly adapted for use in motor vehicles.
This arrangement includes two side plates and a drive band which is disposed between the side plates. The ends of the drive band are fastened to a drive shaft. The drive band is wrapped around the drive shaft to form a loop. The interior of this loop can be exposed to expanding gases discharged from a gas outlet in a manner which expands the loop and draws the drive band off the drive shaft in a manner which induces the drive shaft to rotate.
With this arrangement, in the event of an accident, the safety belt is tensioned through a few centimeters of the safety belt being wound up on a belt reel. The belt reel sits on the drive shaft and is driven by the gas powered drive arrangement. However, for an appreciable amount of belt tensioning to take place, the belt reel, and thus also the drive shaft, must be rotated by a significant amount.
This amount of rotation is only possible if a substantial length of the drive band can wind off the drive shaft and produce a sufficient amount of torque. This requires that the drive band have a substantial length and also requires a sufficiently large space for the drive band loop to expand into. Accordingly, expansion chambers of this type of drive must be of a relatively large. This induces the drawbacks that the device requires a relatively large amount of space when disposed in a motor vehicle or the like, and is accompanied by a relatively large weight penalty.
An underlying object of the invention is to provide a compact drive for a belt tensioner.
According to an embodiment of the invention, a drive of the invention is provided wherein the drive band is fixed to the plates at a position disposed between its ends by a fixing apparatus. The movement of the drive band in the case of its winding off is very limited due to this design. The loop formed by the drive band can therefore no longer expand uniformly in all directions on being exposed to the expanding gas, but is prevented from expanding in at least one direction by the fixing apparatus. The dimensions of the drive chamber can be small, and the space consumed by the drive thus reduced, in the direction in which the expansion of the loop is prevented. In this way, not only the space requirements of the drive, but also the drive weight can be reduced.
Advantageous embodiments of the invention are described in the dependent claims, the description and the drawings.
The fixing apparatus can, for instance, have two fixed elements in a first advantageous embodiment, with the drive band being wound around them. The winding angle around each of the two fixed elements is preferably more than 180xc2x0.
When the drive is actuated, the expanding gas discharged from the gas outlet opening exerts a force onto the drive band. The forces acting onto the drive band are deflected at the fixed elements of the fixing apparatus such that they act in opposite directions and compensate one another. A slipping of the drive band through the fixing apparatus is thus prevented in a simple manner.
It has moreover proved to be particularly advantageous if more than one winding around at least one of the two fixed elements is provided.
In this case, forces acting onto the drive band by the expanding gas result in the windings contracting. The drive band is thus not only prevented from slipping through the fixing apparatus by a mutual compensation of the forces, but is additionally fixed in the fixing apparatus by an increase in the friction of the drive band in the windings.
In an alternative advantageous embodiment, the fixing apparatus is formed by two fixed elements and a moving element, with the fixed elements preferably having a spacing to one another which is somewhat smaller than the expansion of the moving element in its widest region. It is then particularly advantageous if the winding angles of the drive band around the fixed elements in each case amount to somewhat more than 90xc2x0 and if the winding angle around the moving element has somewhat more than 180xc2x0.
If the drive is actuated in this case, then the forces exerted onto the drive band by the expanding gas result in the moving element being pulled in the direction of the two fixed elements by the drive band. If the moving element abuts the two fixed elements, then the drive band is clamped between the moving element and the two fixed elements. The drive band is prevented form slipping through in this way and the desired fixing of the drive band achieved.
In a particularly advantageous constructive embodiment, the elements are substantially formed by bolts whose end surfaces adjoin the plates. A mechanically simple fixing apparatus is provided in this way which ensures reliable operation.
It applies to both alternatives of the fixing apparatus that they can be made without large economic costs and can possibly even be easily retro-fitted into existing drives. A further advantage is that already existing drive bands can be used. The drive band is furthermore used in its one-piece form. A complex assembly of a shorter and a longer drive band section each with one end at the drive shaft and the correspondingly other end at the fixing apparatus is thus not necessary.